We have recently demonstrated in 184 non endurance trained non obese men and women from the Baltimore Longitudinal Study of Aging (BLSA) with normal maximal exercise ECGs that the age-associated decline in maximal oxygen consumption (VO.max) was markedly attenuated after VO.max was normalized for total muscle mass instead of body weight. These results suggested an important role of peripheral (vs central circulatory) factors in the age-related reduction of maximal aerobic performance. A closely related issue is whether the well documented marked increase in VO,max of older endurance trained men relative to that of age-matched peers is mediated primarily by central versus peripheral adaptations. We performed radionuclide ventriculography and VO, measurements during maximal upright bicycle ergometry in endurance trained (mean age = 65 + 5 yr, treadmill VO max = 51 + 4 ml/kg/min) and 6 age-matched sedentary (treadmill VO2max = 31 + 4 mi/kg min) men.